feat: visual identity, theme toggle, custom CSS, loading states (M4)

Grounds the visual design in the product's own subject matter (vital-
signs monitoring) rather than generic dark-mode defaults: a signature
"pulse" green tied to healthy status (the accent color and the "up"
status color are the same hue - a steady green pulse reads as
healthy, same convention as a cardiac monitor), a small ECG-trace
brand mark, and a restrained pulsing-glow animation on healthy status
dots (prefers-reduced-motion respected) as the one deliberate motion
touch. Typography moves off the bare system-ui stack: IBM Plex Sans
for UI text, IBM Plex Mono with tabular numerals for metric values
(CPU/mem/uptime/latency), Space Grotesk used once for the wordmark -
chosen partly for IBM Plex's own systems-monitoring heritage.

Functional additions:
- Light/dark toggle, independent of config.yml's theme.mode, persisted
  to localStorage with a blocking inline script to avoid a flash of
  the wrong theme on load; auto mode now genuinely follows
  prefers-color-scheme instead of hardcoding dark.
- theme.customCssPath support via a new route handler that serves a
  user-mounted CSS file at runtime (can't be a build-time import,
  hence the targeted no-css-tags lint suppression), with a path-
  traversal guard since it's still reading from disk on every request.
- Loading-skeleton state for widgets awaiting their first WebSocket
  result, distinct from both the error state and genuine no-data.

Verified visually in Chrome: theme toggle switches instantly in both
directions, a live docker widget's status dot and monospace metrics
render correctly in both palettes, and config hot-reload still adds a
new widget without a page refresh.

components/widgets/database/Widget.tsx

@@ -26,7 +26,9 @@ export function DatabaseWidget({ widget, widgetId }: { widget: DatabaseWidget; w
       <span className="text-xs text-fg-muted">{ENGINE_LABEL[widget.engine]}</span>
       {errorMessage && <span className="text-xs text-status-down">{errorMessage}</span>}
       {data?.uptimeSeconds != null && (
-        <span className="text-xs text-fg-muted">Uptime: {formatUptime(data.uptimeSeconds)}</span>
+        <span className="text-xs text-fg-muted">
+          Uptime: <span className="font-mono tabular-nums text-fg">{formatUptime(data.uptimeSeconds)}</span>
+        </span>
       )}
     </div>
   );